Method of making porous metal molds



2,835,572 Patented May 20, 1 958 METHOD OF MAKING ronous METAL MOLDS No Drawing. 7 Application July-30, 1954 Serial N0. 546,986

1 (Claim. (Cl. 75-300) The invention relates to the product-ion of porous molds and more particularly to uniformly porous molds suitable for vacuum forming of plastic sheets.-

Many methods have been developed for producing molds. For example, plaster molds cast and dried in the usual manner can be used for vacuum forming. Solid castings of metal or plastic can be made porous by drilling holes or inserting pins therein during casting and removing the pins after the casting. Another technique, while not normally used for the purpose of making molds for vacuum forming, can be used to practice the invention. This involves making porous metal filters as practiced in the field of powder metallurgy by heating either compacted or loose metal powders in a mold of the desired shape to a temperature range of 1600 F. in neutral or reducing atmosphere. Interditfusion of the alloying components in the powders at such a relatively high temperature bonds the particles at their points of contact.

A further method employed in the foundry industry called shell-molding provides thin porous molds suitable for casting metals, said molds being formed from sand and resin. Specifically, a metal plate pattern is coated with a separating agent and then heated to 350- 400 F. At this point, a mixture of sand and phenolic resin is dumped onto the pattern; the resin melts bonding the sand together. Thickness of the pattern thus formed is determined by the residence time of the mix on such pattern. After a set interval, the pattern is inverted and the excess material removed. Then the pattern with its bonded blanket is cured at a temperature of 650-700 F., cooled and finally, the thin shell reproduction is separated from the pattern.

All the methods for making molds mentioned hereinbefore have disadvantageous features. Plaster molds, while porous for crude vacuum formings are not sufficiently porous for detailed reproductions. In order to increase the porosity of such. molds, additional holes must be drilled in the plaster. Further, the thermal properties of plaster are very poor and surface degradation and cracking result from calcination and thermal shock. Mechanical insertion of holes in solid castings is very tedious work and in many instances results in the production of imperfect molds. The techniques used in industrial production of porous metal filters and shell molding require expensive equipment and high temperatures.

One object of the invention is to provide a method for the production of porous molds of metallic powder suitable for vacuum forming.

Another object of the invention is to provide inexpensive, structurally porous molds for vacuum forming.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description.

The present invention is directed to a method that overcomes the inherent short-comings of resin-bonded porous molds. As mentioned hereinbefore resin molds have poor thermal conductivity and result in castings having relatively low bonding strength. Molds made from metal will obviously solve the problem. However, present methods for making metal molds require high temperatures, special graphite molds and special processing equipment. Several methods to be detailed hereinafter provide structurally porous molds for use in vacuum forming. The methods involve the bonding of metallic powders using low temperatures.

Method A" To a powdered metal, such as bronze powder, is added a solder powder, having the ratio of 67% tin, 33% lead, and a dry powder flux of aluminum chloride. The specific materials and ratios mentioned are by way of illustration and not deemed to be critical in practicing the invention. The mixture is intimately mixed and then cast onto a suitably prepared plaster-starch mold. Said mold containing the mixture cast thereon is treated to a temperature of approximately 400 F. in an oven and kept at that temperature for several hours. After bonding is complete, the casting is then cooled and the finished metal mold is removed from the pattern by disintegrating the plaster-starch mold in water. If a corrosive flux is used, the mold should be washed to dissolve out the remaining flux but where a non-corrosive flux is employed, no washing is necessary.

M ethod B Method C A shell mold is formed by simultaneously spraying a mixture of brass and solder powder, and a flux solution onto a plaster-starch pattern. Such a spraying operation can be accomplished by using two spray guns and adjusting their rate of flow. The flux solution which can be zinc or aluminum chloride or a resin in alcohol solution acts as a temporary binding agent to keep the metal powders in a shell form. Said sprayed assembly is then introduced into an oven at approximately 400 F. and maintained at that temperature until bonding is complete. The mold is then cooled and treated as in Method A.

Method D A mixture of powders, one high-melting and the other low-melting, with or without a flux (solid or resin powder mixed with metal) are flame sprayed onto a plasterstarch pattern. The rate of powder feed and the temperature of the flame are adjusted so that only the lower melting metal melts and acts to bind the higher melting metal into a porous reproduction. It is to be observed that in this method, the choice of powders need not be restricted to those that can be heat treated at relatively low oven temperatures, but any metals or alloys that have an aflinity for each other can be used.

Method E A metal powder is sprayed with a liquid such as water to form a thin shell on a plaster-starch pattern; The entire assembly is introduced into an electroplating bath in a horizontal position and the powder bonded together by electrolytic action to form a thin porous shell repro- 3. duction of the plaster mold. The shell is then removed from the pattern in the usual manner.

The methods as described hereinbefore are particularly adapted for the preparation of porous metal molds suitable for vacuum forming. I Existing industrial methods for the production of porous metal filters are not practical or economically adaptable for the preparation of large complex shapes. It can be seen that the invention as described is simple and can be carried out with standard inexpensive equipment presently used for plaster materials. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

What is claimed:

"A process for making porous molds of metallic powder for use in vacuum forming comprising providing a plasterst'arch pattern, mixing bronze powder with a solder powder having a ratio of 67% tin, 33% lead, and pow- References Cited in the file of this patent UNITED STATES PATENTS 1,617,166 Schoop Feb. 8, 1927 1,654,509 Claus Dec. 27, 1927 1,950,604 Fitzgerald Mar. 13, 1934 2,082,354 Reichmann June 1, 1937 2,227,308 Hildabolt Dec. 31, 1940 2,250,099 Hensel July 22, 1941 2,250,246 Axline July 22, 1941 2,262,072 Vaughn Nov. 11, 1941 2,280,864 Stossel Apr. 28, 1942 2,390,160 Marvin Dec. 4, 1945 2,698,990 Conant et a1. Jan. 11, 1955 2,744,011 Samuel et al. May 1, 1956 

